1. Field of the Invention
The invention pertains generally to electric machines. More specifically, the invention relates to the class of electric machines using no permanent magnets and operating solely on the principle of magnetic attraction between an array of soft magnetic parts that are mounted on a fixed base (usually referred to as “stator”) and another array of soft magnetic parts that can move with respect to the fixed base (usually referred to as “rotor” or “translator”), where only the array on the fixed base contains electrical windings connected to a power converter and controlled by a logic unit.
In order to ascertain accurately the place of the machine that constitutes the subject of this invention within the aforementioned class of machines, a brief exposé on the specificity of these machines is presented in this section.
This class of machines has two essential features: it employs no permanent magnets in either the stator or the rotor magnetic paths, and has no electrical windings on the rotor. The first feature ensures that such machines are free from the detrimental effects of the electromagnetic interaction that may occur between the magnetic field of the permanent magnets and the stator windings when the rotor is spinning. This is achieved by using exclusively soft magnetic materials for the magnetic circuits.
The second feature allows a very simple and robust construction of the rotor and eliminates the need for slipping electrical contacts for the rotor windings.
An essential trait of such machines consists in the exclusive use of the soft magnetic attraction force between the stator and rotor magnetic parts in order to operate (magnetic repulsion between these parts is not possible in the absence of permanent magnets). The soft magnetic attraction force is generated by applying electric current pulses in the stator windings when the inductance of these windings can vary (increase or decrease) as a result of rotor motion. When the current pulses are cut off, the magnetic attraction force is no longer generated (this being the meaning of the term “soft magnetic attraction” in the context of this presentation). In literature, the machines using this mode of operation are generally referenced as “switched reluctance machines”, which the authors of this invention consider to be a misnomer (the reason being that the magnetic reluctance of such machines is a physical property that cannot be “switched” on or off, but can be varied rather gradually; the term “variable reluctance machines” would be more appropriate to reference this kind of machines).
A functional analysis of this type of electric machines shows that they operate in a sequential mode by constantly “chasing” the positions of increasing stator inductance caused by the rotor motion (in motoring mode) or the positions of decreasing stator inductance caused by the rotor motion (in generation mode). To operate continuously, the windings of these machines need to be connected to the electrical power supply only when this condition is met and disconnected when the condition is not met. Therefore, such machines cannot operate by plugging them directly into an electrical power line. For instance, a permanent DC power line feed would end up locking the rotor in the nearest aligned position with some of the stator poles until the feed is removed. This is due to the fact that the position of alignment of the active stator poles with the rotor poles is a position of minimum reluctance of the magnetic system, which is always a position of stable equilibrium. Consequently, these machines can work only in sequential feed mode. To achieve this, their operation requires an electrical interface to the power line (a power converter) and a logical interface to provide the correct time sequence for the electrical interface operation (a logic unit). Although these three components (the basic machine, the power converter and the logic unit) are physically separate parts, all three are indispensable to operate an autonomous (stand-alone) electric machine based on the soft magnetic attraction principle.
Due to the sequential operation mode, these machines can be treated essentially as integrators of discrete actions. A discrete action is produced, for instance, when a machine winding (located on the stator) is fed an electric current pulse. The magnetic parts of the machine affected by this pulse constitute an active element. If a machine is comprised of multiple interconnected active elements, the superposition of their discrete actions along the time axis constitutes a time integration function performed by the machine. For this type of machines, an active element consists of a pair of stator-rotor poles that are interacting via the magnetic attraction force produced in the air gap between them. To obtain a continuous output, this type of machine is divided structurally into several groups of active elements called “phases” operating in sequential time windows, which can produce discrete actions aligned more or less seamlessly along the time axis. All the active elements comprising a phase are activated simultaneously and therefore create discrete actions that are always situated in the same time window.
An essential feature of the phases that comprise a stand-alone electric machine is that they are functionally co-dependent during operation. This means that the operation of any of the constituent phases depends on the correct operation of the other phases in order to fulfill its function, otherwise the machine as a whole cannot operate as intended. If a phase of a stand-alone machine stops working, the machine might still operate at reduced capacity but with de-rated parameters (high-speed machines), or might stop working altogether (low-speed high-torque machines). This aspect is important in order to make a clear distinction between stand-alone machines and systems of machines. A system of machines comprises two or more stand-alone machines (or their main parts) assembled together in a specific arrangement in order to achieve a certain functionality (higher power, smoother operation, multiple loads handling, redundancy, etc.) that an existing stand-alone machine cannot provide. If one of the machines in the assembly stops working, the rest of the assembly may continue to work unimpeded. By contrast, a stand-alone machine is not reducible to a simpler structure without altering fundamentally its operation.
The vast majority of the machines based on the soft magnetic attraction principle are based on the rotary topology, where the moving part is a spinning rotor unit. Because of their shape, these machines have cylindrical symmetry. Consequently, the magnetic force developed in the air gap between the stator and rotor poles can have components along the axial, radial and tangential directions of the machine. The only component that can be exploited for practical use in the existing machines is the tangential one, since only a tangential force can generate a torque that cause the rotor to spin, but this component can never be obtained alone in a rotary topology. In order to remove the effects of the other two components (which are torque-neutral), the machine designer must devise solutions to either reduce these components to negligible values or to use the machine symmetry to cancel them out. From this perspective, there are two types of machines that can be built on the rotary topology: the radial field type and the axial field type. The radial field machine has a radial magnetic field in the air gap, which produces a powerful radial force that causes the stator poles to attract the rotor poles radially when activated. This force cannot be used, since it is torque-neutral, and can be cancelled effectively if the machine is designed to activate simultaneously stator-rotor pole pairs positioned in diametrically opposite locations. This is the reason why this type of machine has always an even number of stator poles. The axial component, also torque-neutral, can be annulled by positioning the rotor inside the stator symmetrically with respect to the axial direction of the machine.
The axial field machine has an axial magnetic field in the air gap, which produces a powerful axial force that causes the stator poles to attract the rotor poles axially when activated, but it is torque-neutral. This force can be cancelled either by using axially-loaded bearings or by using a pair of stator units positioned symmetrically on both sides of a common rotor unit. The torque-neutral radial component is not generated in this type of machine, so there is no need to cancel it.
In light of the exposé presented above, the machine that constitutes the subject of this invention is a stand-alone radial field machine. The main objective of the invention is to produce an ingenious machine structure that can be analyzed and designed using digital modeling, can operate based exclusively on digital pulses, can execute compound types of motion (such as rotation and translation), and can reach the highest energy efficiency attainable with the materials and technology available today.
In order to achieve these goals, the machine is based on the matrix calculus and is structured (as will be detailed in a separate section) as an assembly of two coaxial cylindrical matrices of active elements, one fixed and one free to move about the common axis. The active elements are operated sequentially by a multi-output electrical power converter interfaced with a logic control module. The subject of the invention belongs to the category of fully reciprocal electric machines, therefore this machine may be operated in either motoring or generation mode without any hardware adjustment.
2. Description of Prior Art
The best known approaches in prior art that are based on the soft magnetic attraction principle are those pertaining to the class of multi-phase radial field electric machines referenced previously as “switched reluctance machines” (SRM) and the class of multi-phase radial field electric machines known as “stepper motors” (SM). The multi-phase machines belonging to these classes are built with all the phases located on common stator and rotor units.
The development of such machines in rotary form has settled for the type of multi-phase electric machines with a certain number of stator poles per phase (the simplest version having two diametrically opposed poles), where a single stator unit comprises all stator phases distributed more or less evenly on its circumference and a single rotor unit comprises all rotor poles distributed in similar fashion. In order to obtain a continuous rotational motion, the stator and rotor units comprise a different number of poles and the stator windings are activated in a certain temporal sequence by an external power converter, so that only the stator poles of the same phase can come into radial alignment with certain rotor poles at any given time. The ratio between the number of stator poles and the number of rotor poles is a key machine parameter and is usually determined by the number of machine phases. These machines are inherently variable speed machines and cannot be operated directly from the power line, as shown previously. This is the reason why a power converter is always necessary to drive them (in motoring mode) or to convert their electrical output into standard line power (in generation mode). This combination is generally called a “drive”. The correct timing for the activation of their phases by the power converter is either assisted by a position sensor/encoder built into the machine or inferred by means of certain algorithms (sensorless timing) in the more advanced converters. The magnetic attraction force between the stator and rotor poles is produced in the air gap between them and is dependent on their relative position. In operation, the tangential component of the magnetic force produced by the momentarily active phase brings the closest rotor poles from an unaligned position to the aligned position with its stator poles, from which the next phase, becoming active, brings the rotor poles to another aligned position, and so on. A continuous rotating motion of the rotor is therefore obtained by activating the phases sequentially in order to force the rotor unit to spin in the desired direction.
The majority of the machines built on this principle are radial field machines, owing to their simple core construction. The magnetic circuit of the phases can have either a long path (when the phase stator poles are positioned in diametrically opposite locations) or a short path (when the phase stator poles are positioned in adjacent locations). Although the short path solution is more advantageous for many reasons (especially high energy efficiency), in the classic design it has a significant drawback due to the higher magnetic interference between phases and uneven magnetic pull on the rotor due to the inadequate cancellation of the radial component of the magnetic force around the machine circumference. For this reason, most machine designs use the less efficient long path solution.
An in-depth analysis of the current state of the art reveals a series of limitations encountered in the operation of such stand-alone radial field machines. A straight treatise on this type of machines will show both the pros and cons of their operation. Among the pros, the most significant ones are: quasi-independence between the machine phases (which affords fault tolerance), simple rotor construction (which offers high reliability in operation, especially at high rotation speeds), high starting torque, high energy density and relatively inexpensive manufacturing. All these features are a direct consequence of the distinctive principle of operation, which is based exclusively on the soft magnetic attraction force (no permanent magnets required). Among the cons, the most problematic ones are those encountered in motoring mode: uneven torque on the rotor (which causes shaft fatigue, vibrations and acoustic noise), the onset of negative (braking) torque at high rotational speeds (which reduces the energy efficiency) and the low duty cycle achievable per phase due to the extra time required to unload the magnetic energy stored in the air gap (which reduces the output power capability).
To counter some of these problems, most solutions found in the prior art are largely geared towards compensating the basic machine limitations through special power converter features. The most effective of these solutions consists in the current-shaping technique built into the power converter. This feature allows a significant reduction of the torque ripple by altering the waveform of the current pulses fed into the machine phases. This technique can also help minimize the negative torque by altering the cut-off timing of the current pulses. However, the current-shaping technique reduces the phase duty cycle. At any rate, any such technique cannot solve single-handedly all the problems that are inherent to these machines. In addition, this strategy requires expensive power converters or sophisticated feedback loops in the logic unit.
Among the solutions catering to the basic machine features, most are concerned with the refinement of the stator and rotor pole geometry in order to minimize the drawbacks of the standard machine characteristics. There are solutions presenting various pole shapes and profiles (such as the “shark” pole profile) and pole orientations (such as the slanted axial orientation), intended to ameliorate certain basic machine deficiencies. Other approaches found in the prior art that are supposed to tackle the problems at the basic machine level are actually work-around solutions which depart from the irreducible structure of the stand-alone machine, transitioning into the realm of systems of machines. This strategy does not constitute a set of proper solutions to the basic machine problems and can be treated at best as a source of pseudo-solutions when altering the basic machine structure is not effective or not even possible. Moreover, this strategy is not specific to the stand-alone multi-phase radial field machines (the concept of systems of machines represents a broad domain, covering many types of machines).
One of the main advantages claimed by the existing electric machines based on the soft magnetic attraction principle is their insensitivity to the failure of one phase, due to the assumed lack of magnetic coupling (interaction) between phases. While this would be a valid claim in principle, it is based on an assumption nonetheless. In actuality, the phases of these machines are never magnetically independent, since they are built on the same stator and rotor units and inherently share common magnetic paths. Despite the best efforts at the design stage, the phases of these machines still have typically 6% to 15% mutual flux linkages between phases. Therefore, the assumption about the magnetic independence of phases is only a simplification, not a genuine characteristic of these machines, and this will not change as long as the phases of these machines will continue to be built on common stator and rotor units.
The design of various incarnations of the machines in prior art is usually based on more or less sophisticated analogue modeling of the basic machines. Due to the dovetailing of the phases on common stator and rotor units, in prior art there are drastic limitations on the optimization of the basic machine design, mainly owing to the commonality of magnetic paths between adjacent phases. As a consequence, the stator-rotor pole size ratio is limited to a very narrow range of values, which precludes the maximization of the pole duty cycle and results in poor energy density per phase. Moreover, because of these undesirable flux linkages between phases, the analysis of such machines is difficult and devising accurate mathematical models to perform a satisfactory analysis is very laborious. In order to take into account the parasitic inter-phase flux linkages, the models are further complicated by adding second-order flux sources, making the machine analysis even more laborious. Any change in the machine design will lead to a revision of the model and a new analysis must be carried out to assess the machine performance. Due to the difficulty of machine analysis, the embodiment of various designs often fall off the mark significantly when tested against the initial design parameters, and necessary design revisions are usually required to reach the target.